Why pkg-config only print `-l`, but ignored `-I, -L`?

All I tested is on Ubuntu x86_64.I have zlib.pc file in /usr/lib/x86_64-linux-gnu/pkgconfig, the content in this file:

prefix=/usrexec_prefix=${prefix}libdir=${prefix}/lib/x86_64-linux-gnusharedlibdir=${libdir}includedir=${prefix}/includeName: zlibDescription: zlib compression libraryVersion: 1.3Requires:Libs: -L${libdir} -L${sharedlibdir} -lzCflags: -I${includedir}

The environment VAR: PKG_CONFIG_PATH=/usr/lib/x86_64-linux-gnu/pkgconfig

But when I execute:

$ pkg-config --libs --cflags zlib

I got the result:

-lz

There is no -L and -I.

What the fucking problem? drived me crazy. I just wanna keep the -I and -L without any other arguments like --keep-system-cflags --keep-system-libs